This invention relates to an implantable austenitic stainless steel alloy, and in particular to such an alloy and articles made therefrom in which the elements comprising the alloy are carefully selected and the thermal treatment cycles are closely controlled to prevent ordered phases and thus provide a unique combination of biofunctionality (e.g., high yield strength, good ductility, and good low-cycle fatigue resistance and resistance to stress corrosion, cracking, and pitting), and highly improved biocompatible characteristics due to lowered nickel-chromium toxicity.
Austenite generally does not exist at room temperature in plain-carbon and low-alloy steels, other than as small amounts of retained austenite that did not transform during rapid cooling. However, in certain high-alloy steels, such as the austenitic stainless steels and Hadfield austenitic manganese steel, austenite is the dominant microstructure. In these steels, sufficient quantities of alloying elements that stabilize austenite at room temperature are present (e.g., manganese and nickel). The crystal structure of austenite is face-centered cubic (fcc) as compared to ferrite, which has a body centered cubic (bcc) lattice. A fcc alloy has certain desirable characteristics; for example, it has low-temperature toughness, excellent weldability, and is nonmagnetic. Because of their high alloy content, austenitic steels are usually corrosion resistant. Disadvantages of the austenitic steels are their relative high costs, their susceptibility to stress-corrosion cracking (certain austenitic steels), the fact that they cannot be strengthened other than by cold working, interstitial solid-solution strengthening.
The austenitic stainless steels (e.g., type 301,302, 303, 304, 305, 308, 309, 310, 314, 316, 317, 321, 330, 347, 348, and 384) generally contain from 6 to 22% nickel to stabilize the austenite microstructure at room temperature. They also contain other alloying elements, such as chromium (16 to 26%) for corrosion resistance, and smaller amounts of manganese and molybdenum. The widely used type 304 stainless steel contains 18 to 20% Cr and 8 to 10.5% Ni, and is also called 18-8 stainless steel. The yield strength of annealed type 304 stainless steel is typically 290 MPa (40 ksi), with a tensile strength of about 580 MPa (84 ksi). However, both yield and tensile strength can be substantially increased by cold working. However, the increase in strength is offset by a substantial decrease in ductility, for example, from about 55% elongation in the annealed condition to about 25% elongation after cold working.
Some austenitic stainless steels (type 200, 201, 202, and 205) employ interstitial solid-solution strengthening with nitrogen addition. Austenite, like ferrite, can be strengthened by interstitial elements such as carbon and nitrogen. However, carbon is usually excluded because of the deleterious effect associated with precipitation of chromium carbides on austenite grain boundaries (a process called sensitization). These chromium carbides deplete the grain-boundary regions of chromium, and the denuded boundaries are extremely susceptible to corrosion. Such steels can be desensitized by heating to high temperature to dissolve the carbides and place the chromium back into solution in the austenite. Nitrogen, on the other hand, is soluble in austenite and is added for strengthening. To prevent nitrogen from forming deleterious nitrides, manganese is added to lower the activity of nitrogen in the austenite, as well as to stabilize the austenite. For example, type 201 stainless steel has composition ranges of 5.5 to 7.5% Mn, 16 to 18% Cr, 3.5 to 5.5% Ni, and 0.25% N. The other type 2xx series of steels contain from 0.25 to 0.40% N.
Another important austenitic steel is austenitic manganese steel. Developed by Sir Robert Hadfield in the late 1890s, these steels remain austenitic after water quenching and have considerable strength and toughness. A typical Hadfield manganese steel contains 1 to 14% Mn, 0.95 to 1.4% C, and 0.3 to 1% Si. Solution annealing is necessary to suppress the formation of iron carbides. The carbon must be in solid solution to stabilize the austenite. When completely austenitic, these steels can be work hardened to provide higher hardness and wear resistance. A work hardened Hadfield manganese steel has excellent resistance to abrasive wear under heavy loading. Because of this characteristic, these steels are ideal for jaw crushers and other crushing and grinding components in the mining industry. Also, Hadfield manganese steels have long been used for railway frogs (components used at the junction point of two railroad lines).
AMSI Types 304L, 316L, 321 and 347 stainless steels are austenitic, chromium-nickel and chromium-nickel-molybdenum stainless steels having the following compositions in weight percent:
Source: METALS HANDBOOK RTM. Desk Edition; Chapt. 15, pages 2-3; (1985). The AMS standards for these alloys restrict copper to not more than 0.75%.
The above-listed chromium-nickel and chromium-nickel-molybdenum stainless steels are known to be useful for applications which require good non-magnetic behavior, in combination with good corrosion resistance. One disadvantage of the series 300 stainless steels is their potentially poor biocompatibility, due principally to nickel-chromium toxicity. Therefore, this present invention alloy can be useful in clinical indications because it can provide improved biocompatibility due to lowered nickel-chromium percentages and therefore less toxicity.
Given the foregoing, it would be highly desirable to have an austenitic stainless steel that provides better biocompatibility than is provided by the known austenitic stainless steels.
The invention generally relates to an implantable austenitic stainless steel alloy that provides better biocompatibility than is provided by the known austenitic stainless steels. One application for the present invention is to use the austenitic stainless steel alloy with increased biocompatibility for fabricating intravascular stents. Typically stents are fabricated from a variety of stainless steels, with the 316 series representing a large percentage of the stainless steel used to fabricate currently marketed stents. The typical composition of 316 series implant grade stainless steel is shown in Table I.
While the 300 series of stainless steel has several characteristics, such as strength, flexibility, fatigue resistance, biocompatibility, etc. rendering it a good material to make an intravascular stent, one significant disadvantage of 316 series stainless steel, as well as other 300 series of stainless steel, is that they have relatively high nickel-chromium percentages and therefore have the potential for toxicity and poor biocompatibility. A need has arisen to modify the stainless steel composition so it has improved biocompatible properties while at the same time, maintaining those characteristics which render it as a material of choice for implants.
Modified stainless steel of the 300 series for increasing biocompatible characteristics could be produced by creating alloys containing varying amounts of elements that have dense mass and biocompatible characteristics. The chemical make-up of standard series 300 stainless steel, using series 316 as an example, along with the possible chemical ranges of various such alloys are shown on the following Table II. One example of this alloy employs molybdenum in the composition while another example does not use the molybdenum element.